1. Field of the Invention
The present invention relates to a base station facility conducting a cellular mobile communication using a code division multiple access (CDMA) operation, and in particular, to a base station facility capable of autonomously determining a spreading code to be used for identification of a base station associated therewith when the base station is first installed.
2. Description of the Related Art
In a mobile communication employing the CDMA operation, the spectrum of information signals are spread into a band sufficiently broader than an inherent information band width thereof in accordance with a spreading code for transmission thereof to thereby conduct a spectrum spread communication. The spreading codes are assigned to users to achieve multiple accesses, namely, code division multiple accesses (CDMA).
Moreover, in a cellular mobile communication system adopting the CDMA operation, a spreading code is set to each base station as an identification code thereof such that a mobile station receives, via a shared control channel, information transmitted from the base stations using the respective spreading codes to identify one of the spreading codes for which the highest correlation output is attained, and resultantly recognizes a cell to which the mobile station belongs.
Additionally, there has been considered code allocation in which a long code is set to each base station and a short code is assigned to each user such that a spreading code is created by multiplying the short code by the long code. Reference is to be made to the "Channel Code Allocations for DS-CDMA Mobile Radios", Umeda et al., 1995 IEICE Conference B-426.
In the CDMA cellular mobile communication system, it is possible to repeatedly use the same frequency in all cells. However, when assigning spreading codes to be set to the respective base stations, the code assignment is required to guarantee that the base stations are appropriately identified.
For this purpose, when a new base station BS7 is to be installed as shown in FIG. 10, some of the spreading codes which are not used by adjacent base stations BS1 to BS6 and which are available to the base station BS7 are selected as candidates. One of the candidates being used by a base station BSi, which is installed apart from the base station BS7 and of which a received field level is fully low at the installation place of B27, is set as the spreading code thereof.
However, in the conventional CDMA cellular mobile communication system, a manager of the system designs assignment of stations to determine the spreading code for each base station. Therefore, each time a new base station is installed, a large amount of manpower and a long period of time are required to determine a spreading code for the pertinent base station.
On the other hand, "Autonomous Frequency-Selection and Interference-Detection Algorithm for Underlaid Micro/Pico Cell System", Kakinuma et al., 1995 IEICE Conference B-379 discloses an algorithm for selecting a transmitting frequency to be used for a small base station on the basis of signal-level measurement in mobile stations and the base station. Further, in accordance with the JP-A-7-222232, there has been described a radio channel allocating method in which the call loss probability as well as the chance of occurrences of interference are not deteriorated even when any change takes place in a radio propagation apparatus is due to, for example, installation of a new radio base station in accordance with an autonomous distributive allocation method. In this method, each radio channel is assigned with a priority level. Namely, there is obtained a weight which changes in association with results of allocatability or assignability decision for the radio channel in the past. Specifically, the weight alters in relation to the number of assignability decisions in the past relative to the decision of assignability at the current point of time. Channel assignability is determined for the radio channels in a descending order of priority levels assigned thereto. In the article, there has been described a method of determining assignability of a radio channel in which a received level or a ratio between a desired wave and an interference wave is measured in the pertinent channel such that the channel is to be allocated when the measured value is equal to or more than a threshold value. However, the correlation of the spreading code is essential in the CDMA operation and the allocation determining method above does not necessarily lead to an optimal state to receive signals.